1. Field of the Invention
This invention relates generally to ultrasonic cleaning and liquid processing methods and apparatus and other uses involving two or more piezoelectric transducers, and relates more particularly to improving performance by using ultrasonic energy at multiple frequencies.
2. Description of the Relevant Art
Ultrasonic devices are used in a variety of processes, including cleaning, emulsifying, and dispersing components or parts in a liquid medium, and other applications such as metal welding, plastic joining, and wire bonding. All these devices and processes use ultrasonic transducers to supply ultrasonic frequency sound waves to a liquid or solid medium.
Cleaning parts in a liquid medium is one common use of ultrasonics. Cleaning with ultrasonics uses ultrasonic waves to generate and distribute cavitation implosions in a liquid medium. The released energies reach and penetrate deep into crevices, blind holes and areas that are inaccessible to other cleaning methods.
Ultrasonic waves are -pressure waves formed by actuating the ultrasonic transducers with high frequency, high voltage current generated by electronic oscillators (typically referred to as power supplies or generators). A typical industrial high power generator produces ultrasonic frequencies ranging from 20 to 300 kHz or more. Ultrasonic transducers typically include piezoelectric (PZT) devices that expand and contract when subjected to the oscillating driving signals supplied by generators. The transducers are normally mounted on the bottom and/or the sides of the cleaning tanks or immersed in the liquid. The generated ultrasonic waves propagate perpendicularly to the resonating surface. The waves interact with liquid media to generate cavitation implosions. High intensity ultrasonic waves create micro vapor/vacuum bubbles in the liquid medium, which grow to maximum sizes proportional to the applied ultrasonic frequency and then implode, releasing their energies. The higher the frequency, the smaller the cavitation size.
The energy released from an implosion in close vicinity to the surface collides with and fragments or disintegrates the contaminants, allowing the detergent or the cleaning solvent to displace it. The implosion also produces dynamic pressure waves which carry the fragments away from the surface. The cumulative effect of millions of continuous tiny implosions in a liquid medium is what provides the necessary mechanical energy to break physically bonded contaminants, speed up the hydrolysis of chemically bonded ones and enhance the solubilization of ionic contaminants.
In general, at low frequencies (20–30 kHz), a relatively smaller number of cavitations with larger sizes and more energy are generated. At higher frequencies, much denser cavitations with moderate or lower energies are formed. Low frequencies are more appropriate for cleaning heavy and large-size components, while higher frequency (60–80 kHz) ultrasonics is recommended for cleaning delicate surfaces and for the rinsing step.
In some applications it is advantageous to use multiple transducers operating at different frequencies in combination. See, for example, U.S. Pat. No. 6,019,852 and U.K. Patent 1,488,252. These patents disclose cleaning apparatus with rectangular grids of two different frequency transducers, separately driven by two power supplies or generators.